The present invention relates to an image forming apparatus for outputting an image by transferring a developer image from an image carrier to a transfer medium. More specifically, the invention relates to an image forming apparatus wherein the transfer medium is held on a transfer belt that is made to run while touching the surface of a photosensitive drum, and wherein a transfer bias is applied to the transfer belt from inside the region defined by the belt so as to transfer a toner image formed on the image carrier to the transfer medium held on the transfer belt.
In recent years, digital technology has made rapid progress in the technical field of image forming apparatuses of electrophotographic type, and many digital copying machines have come into wide use. A digital copying machine comprises: a scanner for converting light reflected by a document into digital signals and outputting the digital signals; an exposure section for exposing the surface of a photosensitive drum with laser beams corresponding to the digital signals, thereby forming an electrostatic latent image on the surface of the drum; a developing section for supplying toner, which serves as a developer, to the electrostatic latent image, thereby forming a toner image on the surface of the drum; a transfer belt which is made to run while touching the surface of the drum; and a transfer member for transferring the toner image from the surface of the drum to a recording sheet, which serves as a transfer medium and which is conveyed while being held on the transfer belt.
Many of the digital copying machines of this type employ a reversal development system. In a machine employing this system, the photosensitive drum and the transfer member become opposite in polarity in the transfer step. Since, therefore, the recording sheet assumes an opposite polarity to that of the photosensitive drum, it is electrostatically attracted by the photosensitive drum. Therefore, some measures have to be taken to separate the recording sheet from the photosensitive drum after the recording sheet has passed through the transfer region.
Many of the conventional laser printers are comparatively low in process speed, and the photosensitive drums they employ are comparatively small in diameter. Accordingly, recording sheets can be easily separated from the photosensitive drum by utilization of the resilience of the sheets. In order to facilitate the separation of recording sheets, separation means made of a corona charger or the like may be adopted, if so desired. By way of contrast, digital copying apparatuses are comparatively high in process speed and yet employ an organic photosensitive drum of comparatively low sensitivity. The diameter of that drum is inevitably large, and the separation based on the resilience of recording sheets is not easy. In consideration of these problems, some of the existing digital copying apparatuses employ a belt-like photosensitive member in place of the photosensitive drum, and that portion of the belt-like photosensitive member at which a recording sheet is separated therefrom has a comparatively large radius of curvature. Alternatively, some of the existing copying apparatuses employ a belt-like transfer member, and a recording sheet is electrostatically attracted by this belt-like transfer member.
However, these conventional techniques do not ensure easy separation of recording sheets from a photosensitive member while simultaneously maintaining the sufficiently reliable transfer characteristics that realize high-quality images.